linux-kernel - Re: [PATCH v3 00/15] KVM: Introduce KVM Userfault

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Message-ID: <aLrXFWDgDkHqPQda@google.com>
Date: Fri, 5 Sep 2025 05:27:01 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: kalyazin@...zon.com, Paolo Bonzini <pbonzini@...hat.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Yan Zhao <yan.y.zhao@...el.com>, Anish Moorthy <amoorthy@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, wei.w.wang@...el.com, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 00/15] KVM: Introduce KVM Userfault

On Thu, Sep 04, 2025, James Houghton wrote:
> On Thu, Sep 4, 2025 at 9:43 AM Nikita Kalyazin <kalyazin@...zon.com> wrote:
> > Are there any blockers for merging this series?  We would like to use
> > the functionality in Firecracker for restoring guest_memfd-backed VMs
> > from snapshots via UFFD [1].  [2] is a Firecracker feature branch that
> > builds on top of KVM userfault, along with direct map removal [3], write
> > syscall [4] and UFFD support [5] in guest_memfd (currently in discussion
> > with MM at [6]) series.
> 
> Glad to hear that you need this series. :)

Likewise (though I had slightly-advanced warning from Patrick that Firecracker
wants KVM Userfault).  The main reason I haven't pushed harder on this series is
that I didn't think anyone wanted to use it within the next ~year.

> I am on the hook to get some QEMU patches to demonstrate that KVM
> Userfault can work well with it. I'll try to get that done ASAP now
> that you've expressed interest. The firecracker patches are a nice
> demonstration that this could work too... (I wish the VMM I work on
> was open-source).
> 
> I think the current "blocker" is the kvm_page_fault stuff[*]; KVM
> Userfault will be the first user of this API. I'll review that series
> in the next few days. I'm pretty sure Sean doesn't have any conceptual
> issues with KVM Userfault as implemented in this series.

Yep, Oliver and I (and anyone else that has an opinion) just need to align on the
interface for arch-neutral code.  I think that's mostly on me to spin a v2, and
maybe to show how it all looks when integrated with the userfault stuff.